Plasma processing is used for selective etching of solids or for treating solids with excited molecules, atoms, ions, electrons or their compositions to produce certain changes in the surface or near surface regions of the solids. A plasma source is provided for exciting a working gas flow to turn it into an active form. For example, molecules of the working gas are dissociated into atoms or excited to render them efficient in interacting with the solid material. However, in many cases, for example, in treating crystalline semiconductor materials, it is desirable to avoid the bombardment of the substrate surface by ions and electrons contained in plasma because they produce defects in the near surface region of the semiconductor and may lead to degradation of crystal perfection and correspondingly to deterioration of the parameters of semiconductor devices.
Efforts have been made to passivate semiconductor devices by means of a flow of hydrogen gas onto the surface of the semiconductor material. The desired flow consists of hydrogen atoms at a low energy level. Other forms of hydrogen flow, such as a working gas of hydrogen molecules, has no beneficial effect and may have a detrimental effect.
Prior efforts to produce an atomic hydrogen passivation of semiconductor devices are evidenced by U.S. Pat. Nos. 4,113,514 and 4,224,084, which show a glow discharge device to disassociate the atoms of hydrogen molecules, in order to produce atomic hydrogen, but this technique has little practical value. Also, these patents mention, but do not illustrate, thermal dissociation of H.sub.2 to form atomic hydrogen, as well as electron bombardment of H.sub.2.
In producing atomic hydrogen by dissociation of hydrogen molecules, it is important to (1) achieve a high proportion of atomic hydrogen, as compared with molecular hydrogen, in the resulting particle flow used for passivation, (2) eliminate from the resulting particle flow any ions or electrons that may be produced in the dissociation process, and (3) achieve a high density particle flow, as well as a large area flow, so that large areas of semiconductor material may be passivated at a time.
Another effort to produce an atomic hydrogen passivation of semiconductor devices is evidenced by Omeljanovsky et al., "Hydrogen Passivation of Defects and Impurities in GaAs and InP", Journal of Electron Materials, Vol. 18, No. 6, 1989, pp. 659-70. This article refers to igniting a plasma within a resonant cavity and confining the plasma in a beam extending through an aperture in and into a stainless steel tube by a magnetic field produced by an electromagnet and solenoids. A hydrogen working gas flow at supersonic speeds crosses the plasma beam. This crossing results in the hydrogen molecules dissociation and partial ionization of formed hydrogen atoms. Charged particles are deviated by the magnetic field, while atomic hydrogen keep moving forward and hit the substrate within the tube. However, the result does not reach a desireable level of efficiency in plasma processing of substrates, and is limited to an atomic hydrogen flow of about 3.times.10.sup.17 atoms s.sup.-1.
There is thus a continuing need to develop improved devices and methods for plasma processing of substrates, particularly for providing a neutral, atomic flux for treating semiconductor substrates